A nano-sized tubular aluminum silicate is naturally produced as an imogolite, and an imogolite exists in soil and is primarily produced in soil deriving from volcanic ash. Moreover, a natural imogolite, together with allophane as the related mineral, affects the transfer of nutrients and moisture in the soil and their supply to plants, and also affects the accumulation and residue of hazardous contaminants. The foregoing tubular aluminum silicate has, as its primary constituent elements, silicon (Si), aluminum (Al), oxygen (O) and hydrogen (H), and it is a hydrated aluminum silicate assembled by numerous Si—O—Al bonds, has a tubular shape with an outer diameter of 2.2 to 2.8 nm, an inner diameter of 0.5 to 1.2 nm and a length of 10 nm to several μm, and is a clay constituent naturally distributed in the soil having, as its base material, the deposited volcanic ejecta such as volcanic ash and pumice.
A protoimogolite is a precursor substance of the imogolite, and becomes an imogolite by heating the precursor dispersed in an aqueous solution at approximately 100° C. Thus, the precursor substance before heating and during the process of forming the imogolite is referred to as a protoimogolite. Since a protoimogolite has properties that derive from the imogolite structure, it shows the same peak as imogolite at −78 ppm in a 29Si solid state NMR, and the silicon has a coordination of OH—Si—(OAl)3. Consequently, an imogolite and a protoimogolite have roughly the same adsorption behavior at a relative humidity of 20% or less even regarding water vapor adsorption properties. Although the protoimogolite does not grow to a relatively long tube shape as with a crystalline imogolite, it is considered to accordingly possess the imogolite structure. Accordingly, a protoimogolite possesses a similar adsorbent property as with an imogolite in a low-humidity range. The Si/Al molar ratio upon synthesis of both the imogolite and the protoimogolite is 0.35 to 0.55.
In addition, if the Si/Al ratio is set to 0.7 to 1.0 during the synthesis and synthesized material is heated at 150° C. for 2 days after being subject to desalination treatment, a complex of an amorphous substance and low-crystallinity clay is to be formed. Since this complex is a complex of hydrous amorphous aluminum silicate and clay, it is hereinafter referred to as the “HAS clay” based on a complex of HAS (Hydroxyl-Aluminum Silicate) and clay. Moreover, if the Si/Al ratio is set to 0.7 to 1.0 during the synthesis and imogolite or protoimogolite is heated at 100° C. for 2 days after being subject to desalination treatment, the obtained substance also yields extremely superior adsorption performance of water vapor and carbon dioxide. Since this substance becomes a complex of hydrous amorphous aluminum silicate and clay if it is heated at 100° C. for 30 days or more, it is hereinafter referred to as the “HAS clay precursor” (refer to Patent Documents 1 and 2).
The foregoing unique shape and physical properties of an imogolite as the nano-sized tubular aluminum silicate, a protoimogolite as its precursor, HAS clay and HAS clay precursor are considered to be industrially useful. In other words, since an imogolite, a protoimogolite as its precursor, HAS clay and HAS clay precursor are characterized in being able to adsorb various substances based on their unique microstructure; for instance, their availability as a hazardous contaminant adsorbent, a deodorant, a storage material for gas such as carbon dioxide and methane, and the like has been conventionally mentioned. In addition, since these yield superior water vapor adsorption performance, their application as a heat exchange material for heat pump, a dew condensation protectant, an autonomous humidity control material and the like is also expected.
In particular, since desiccant air conditioning aims to eliminate the moisture content in the air that is introduced from the outside air; it is required to efficiently eliminate the moisture content even from the high humidity air in the summer, and the adsorbent that is sought in the desiccant air conditioning is generally demanded of a high adsorption amount at a relative humidity of roughly 5% to 60%.
Under the foregoing circumstances, there are demands for industrially synthesizing an absorbent in large quantities at a low cost while maintaining the foregoing properties of the tubular aluminum silicate and HAS clay, and adsorbents utilizing the unique pores of the imogolite, amorphous imogolite and protoimogolite, and even the HAS clay and HAS clay precursor have been developed.
Nevertheless, according to the conventional production methods, it was necessary to use a mono-silicate aqueous solution as the starting material in order to produce large amounts of the imogolite and amorphous imogolite since the imogolite does not contain the polymer of Si. Moreover, aluminum sulfate was never used as the starting material of the aluminum source since the Keggin-type polymer of aluminum tridecamer blocks production of imogolite (refer to Patent Documents 3, 4 and 5).    Patent Document 1: Japanese Patent Application No. 2007-336403    Patent Document 2: Japanese Laid-Open Patent Publication No. 2008-103658    Patent Document 3: Japanese Laid-Open Patent Publication No. 2001-064010    Patent Document 4: Japanese Laid-Open Patent Publication No. 2004-059330    Patent Document 5: Japanese Laid-Open Patent Publication No. 2008-179533